I found it important to add the Windows-10 tag to this this question Is there a way to force Windows to use Precision Touchpad drivers rather than Synaptics? because it is mainly an issue of Windows 10.
Except the question was asked before Windows 10 was released. So Windows 10 has nothing to do with that particular author's question.
Since Windows 10 is also more common than Windows 8, I thought this tag would be important if people use search engines to learn about this problem. Is it not?
It does not matter if Windows 10 is more popular than Windows 8. What matters what the author was using at the time the question was asked. Answers should be focused on answering the author's question. A question that was asked May 2015, more than likely, was using Windows 8.1 Update 1 instead of a Windows 10 Insider Preview (1507) build.
If a high quality answer applies to both Windows 8 and Windows 10 then it makes sense to update an existing question's tags to reflect that fact. If that is not the case, then changing the question's tags, should not be done without significant thought on the impact with regards to the existing answers the question has received.
I don't actually see anything that would justify adding the Windows 10 tag to that question. The accepted answer is still accurate and applies to every supported version of Windows. The author never specifically specified they were using Windows 10. The only mention of Windows 10 was in temporary commentary by other users.
I thought the main impact of StackExchange comes from helping many users and not just one. Then the clearer its made where the solution applies, the more people can find it and profit from it.
That is something that is only true in specific situations. If a question was asked 60 days before Windows 10 was released, and the author of the question made no mention they were using Windows 10, then it doesn’t make sense to change the tag that was used to Windows 10.
Besides, just changing the tag of a question, is rarely helpful by itself. In this case the question was asked over 5 years ago. Changing the tag won’t make the question easier to find.
I actually don’t agree with that first edit that added the Windows 10 tag in the first place. I absolutely, think removing the Windows 8 tag, was an unhelpful edit.
The addition of the Windows tag was a compromise, I am willing to accept the judgment of a moderator on that decision, it probably was the correct tag in the first place.